This invention relates to film resistors, and more particularly to film resistors formed by the composite of electrically-conducting oxide and aluminum oxide. In addition, this invention relates to resistor and thermistor devices comprising the composite film resistors.
The Ohm""s law, a fundamental principle of electricity, defines the resistance of a device as the ratio of the voltage to the current in an electrical circuit. There are many factors which will affect the resistance value of a device. An important factor discussed here is the temperature of a device. To investigate the characteristics of resistance versus temperature for a device, a measurement called the temperature coefficient of resistance (TCR) is defined as the variation of resistance over the variation of temperature (TCR=dR/dT). If the resistance value of a device is substantially constant with respect to its ambient temperature, that is, the TCR of the device is nearly zero, it is generally referred to as an electrical resistor device. Electrical resistor devices are essential components widely applied in electrical circuits. The shape of an electrical resistor device could possibly be a bulk, a thick film or a thin film. Typical materials used for electrical resistor devices include alloys such as manganin and constantan, as well as graphite-based materials.
In contrast, a temperature-variant resistor could be referred to as a thermistor. A thermistor is an electrical device with a non-zero and large value,of TCR in order to have sensitive resistance variation in response to a temperature change. Thermistor devices having positive TCR, of which resistance increases as the temperature increases, can be used as temperature sensors. High-purity metals like platinum are typical materials for positive TCR thermistors. However, these metals are not useful at low temperatures, where their TCR will approach zero because of impurity effects. The low-temperature difficulty is overcome by semiconductor thermistors that have negative TCR values. Conventional materials of semiconductor thermistors include carbon and germanium.
Similar to resistor devices, thermistors can be in the shape of a bulk, a thick film or a thin film. However, while a thermistor is incorporated into a compact device such as a temperature sensor, the limitation of the thermistor""s physical geometry becomes critical such that thin film thermistors are needed. Other merits of thin film thermistors include their fast thermal responses because of their small thermal masses. There are many thin film thermistors that have been developed. One of the well-known thin film thermistors is formed of a film having metal precipitates in insulating matrices, for example, a film of platinum particles in alumina. This approach has the drawback in difficulty of control during the formation of the film. Other conventional materials for thin film thermistors include common semiconductors such as doped silicon or germanium, but such thermistors are known to be unsuitable for in-fields measurement for their large magneto-resistance. In a more recent method described in U.S. Pat. No. 5,367,285 to Swinehart et al., a film resistor comprising an alloy of both an electrically-insulating oxide and an electrically-conducting nitride of at least one metal is formed by reactive sputtering. By adjusting the volume ratio of the oxygen-containing gas to the nitrogen gas, a desired oxide to nitride ratio and thus, a desired TCR value, can be achieved. However, this method requires a precise control for the volume ratio of reactive gasses and therefore, is difficult and complicated in the fabrication process.
There is, thus, a need to provide an easy and simple method for manufacturing thin film electrical resistors and/or thermistors which are insensitive to magnetic fields for in-field applications. This invention addresses the need.
The present invention relates to film resistors formed by a composite of both electrically-conducting oxide and electrically-insulating aluminum oxide (Al2O3),wherein the electrically-conducting oxide can be ruthenium oxide (RuO2) or iridium oxide (IrO2). In particular, a resistor device of this invention comprises an insulating substrate, a composite film formed of the electrically-conducting oxide and electrically-insulating aluminum oxide and at least two electrodes formed on the composite film.
The composite film is formed by a film deposition process such as non-reactive sputtering deposition with either one target comprised of the mixture of both oxides, or two separate targets respectively made of pure electrically-conducting oxide and pure Al2O3, in an inert gas. By varying the composition ratio of the electrically-conducting oxide and Al2O3 composite, or by adjusting the process parameters of the film deposition such as the inert gas pressure and the substrate temperature, the composite film can have a desired TCR value. Consequently, either an electrical resistor device or a thermistor device can be formed.
In addition to the above-mentioned advantages, the composite film resistors are mechanically resilient and thermally stable such that they are greatly suitable for practical use.